1. Field of the Invention
The present invention relates to a cathode-ray tube. In particular, the present invention relates to a structure of a ground electrode for reducing an undesired electromagnetic field generated particularly on a front surface of glass panel.
2. Description of the Related Art
FIG. 6 is a partially cut-away perspective view of a general cathode-ray tube (hereinafter, referred to as a xe2x80x9cCRTxe2x80x9d) apparatus composed of a CRT and a deflection yoke. The CRT includes a glass panel 1, a glass covering composed of a funnel part 91 and a neck part 92, an electron gun 93 sealed in the neck part 92, a fluorescent screen surface 96 formed on an inner surface of the glass panel 1, color selection electrodes 97 disposed on the electron gun 93 side of the fluorescent screen surface 96 so as to maintain a predetermined interval, a magnetic shield 98, and a multi-layer film (not shown) having antistatic and antireflection functions, formed on an outer surface of the glass panel 1. The deflection yoke 95 is attached to the periphery of the neck part 92 of the CRT so as to deflect an electron beam 94 radiated from the electron gun 93.
FIG. 7 is a plan view of the glass panel 1, and FIG. 3 is an enlarged cross-sectional view of a peripheral portion of the glass panel 1. A surface treatment film 8 composed of a conductive film 3 and a multi-layer film 2 of insulating layers 4 having an antireflection function and the like is formed on the surface of the glass panel 1. A ground electrode 6 is provided on the surface treatment film 8 so as to be electrically connected with the conductive film 3 through ultrasonic solder (Japanese Laid-Open Publication No. 8-287850). As shown in FIG. 7, in general, a pair of ground electrodes 6 are provided on upper and lower portions of the glass panel 1 outside an effective screen region 9.
The fluorescent screen surface 96 is held at a potential of an anode supplied with a high voltage. Therefore, the glass panel 1 is charged to a high potential, which may have adverse effects such as giving shock to a user and causing electronic equipment in the vicinity to malfunction due to discharge. The conductive film 3 and the ground electrode 6 are provided for the purpose of avoiding such a situation.
In recent years, concerns are rising that an electromagnetic wave generated from the deflection yoke 95 which operates repeatedly at a high frequency and an anode part of the electron gun 93 may have some effect on the bodies of a user and people in the vicinity. In Sweden, as guidelines for a display apparatus for a terminal, MPR (The Swedish National Board for Measurement and Testing) and TCO (The Swedish Confederation of Professional Employees) are issued. These guidelines stipulate the suppression of a leakage electric field generated by a display apparatus. According to the most strict TCO guideline, it is required to define a leakage electric field to be 1.0 V/m or less with respect to an alternating electric field in a VLF (Very Low Frequency) band (i.e., 2 kHz to 400 kHz) at a position 30 cm away from the front surface of a glass panel.
As a technique for satisfying the stipulation of the TCO guideline, for example, Japanese Laid-Open Publication No. 10-3868 discloses that a transparent conductive film with a high refractive index having a surface resistance of 9xc3x97102xcexa9/xe2x96xa1 or less is formed on the outer surface of a glass panel, and a plurality of terminals electrically connected with the conductive film are disposed on two or four sides of the glass panel. Furthermore, Japanese Laid-Open Publication No. 10-233180 discloses that a conductive film with a surface resistance of 1xc3x97103xcexa9/xe2x96xa1 or less and an antireflection film (hereinafter, referred to as an xe2x80x9cAR filmxe2x80x9d) are formed on a transparent substrate, the resultant substrate is attached to the outer surface of a glass panel, and an electrode is provided on the outermost layer so as to be grounded in an electrical circuit manner.
In both of the above-mentioned prior art, an electric potential induced on the glass panel is allowed to dissipate to a ground surface on the circuit through the electrode on the outermost surface of the glass panel, whereby it is attempted to prevent the surface of the glass panel from reaching a high potential.
In order to suppress sufficiently a leakage electric field generated on the surface of a glass panel from a deflection yoke and an anode line by using the above-mentioned prior art, the resistance of the conductive film formed as a lower layer of the AR film on the glass panel is required to be about 1xc3x97103xcexa9/xe2x96xa1 or less. However, in order to form a conductive film with a low resistance, a special material containing silver and platinum must be coated by spin coating or sputtering. This increases the production cost. If it is attempted to satisfy the stipulation of the TCO guideline, using a conductive film having a resistance of 1xc3x97103xcexa9/xe2x96xa1 or more, it is required to separately provide a cancel circuit for canceling a leakage electric field, which increases the cost and the number of steps on a circuit side.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a structure of a glass panel that has an effect of sufficiently suppressing a leakage electric field even using a conductive film having a resistance of 1xc3x97103xcexa9/xe2x96xa1 or more.
A cathode-ray tube of the present invention has a structure in which a surface treatment film including an antireflection film layered on a conductive film with a surface resistance of 1xc3x97103xcexa9/xe2x96xa1 or more is provided on a surface of a glass panel, and a ground electrode electrically connected with the conductive film and grounded is formed on the surface treatment film, wherein a surface area of the ground electrode is 500 mm2 or more. By setting the surface area of the ground electrode (i.e., a contact area between the ground electrode and the conductive film) at a predetermined value or more, a leakage electric field can be suppressed to 1.0 V/m or less with respect to an alternating electric field in a VLF band.
Furthermore, it is preferable that the ground electrode is provided in a band shape so as to be substantially parallel to an outer edge of the glass panel outside an effective screen of the glass panel, and the ground electrode has a length of 100 mm or more and a width of 5 mm or more. Because of this, a space of the glass panel outside the effective screen can be efficiently used.
Furthermore, it is preferable that the ground electrode is provided on a side where at least an anode terminal is provided among outer edges of the glass panel. The anode terminal is supplied with the highest voltage and is one of the main sources for causing a leakage electric field. Therefore, by providing the ground electrode in the vicinity of the anode terminal, a leakage electric field can be efficiently suppressed.
Furthermore, it is preferable that the ground electrode is formed by ultrasonic solder. If ultrasonic solder is used, the ground electrode can be formed directly on the surface treatment film without peeling off the surface treatment film.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.